Update default codegen-units
Update default codegen-units

It's been 16 since rustc 1.24.
tshepang/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo,nipunn1313/cargo,rust-lang/cargo,vojtechkral/cargo,tsurai/cargo,nipunn1313/cargo,tsurai/cargo,eminence/cargo,rust-lang/cargo,vojtechkral/cargo,tshepang/cargo,tsurai/cargo,rustyhorde/cargo,vojtechkral/cargo,eminence/cargo,tsurai/cargo,eminence/cargo,tshepang/cargo,tsurai/cargo,vojtechkral/cargo,rustyhorde/cargo,rust-lang/cargo,nipunn1313/cargo,rustyhorde/cargo,rust-lang/cargo,tshepang/cargo